President Obama implored supporters on Monday for donations to help Democrats in the 15 Senate seats he said were "up for grabs" on Nov. 2.

The president painted a picture of an expansive map of competitive Senate races in a fundraising e-mail for the Democratic Senatorial Campaign Committee (DSCC).

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"I'm counting on the DSCC to win the 15 Senate seats still up for grabs over these last 15 days," Obama wrote, asking for more than $900,000 in donations before Thursday to finance a get-out-the-vote "blitz" over the weekend.

Republicans need to pick up a net gain of 10 Senate seats on Election Day in order to win back a majority in the Senate, and make Sen. Mitch McConnellAddison (Mitch) Mitchell McConnellDavid Axelrod after Ginsburg cancer treatment: Supreme Court vacancy could 'tear this country apart' Pelosi asks Democrats for 'leverage' on impeachment Democrats press FBI, DHS on response to white supremacist violence MORE (R-Ky.) the majority leader.

The nonpartisan Cook Political Report lists 11 races as "toss up" contests, nine of which are currently controlled by Democrats. Republicans are also seen by Cook as being well-positioned to pick up Democratic seats in Indiana, Arkansas and North Dakota.

Obama said in his appeal to Democrats that the 15-seat playing field he described — and it's not clear which races exactly make up that list — will be "going right down to the wire." 

The president will be lending his support for vulnerable Democrats, too, with several days of travel and fundraising this week. Obama will attend a DSCC fundraiser in suburban Washington DC on Monday night, and travel to Washington on Thursday for an event for Sen. Patty MurrayPatricia (Patty) Lynn MurrayOvernight Health Care: Planned Parenthood to leave federal family planning program absent court action | Democrats demand Trump withdraw rule on transgender health | Cummings, Sanders investigate three drug companies for 'obstructing' probe Democrats demand Trump officials withdraw rule on transgender health The Hill's Morning Report - Progressives, centrists clash in lively Democratic debate MORE (D). He'll attend fundraisers for Sen. Barbara BoxerBarbara Levy BoxerOnly four Dem senators have endorsed 2020 candidates Hispanic civil rights icon endorses Harris for president California AG Becerra included in Bloomberg 50 list MORE (D-Calif.) and Senate Majority Leader Harry ReidHarry Mason ReidNo, it is not racist to question birthright citizenship McConnell rejects Democrats' 'radical movement' to abolish filibuster Harry Reid: 'Decriminalizing border crossings is not something that should be at the top of the list' MORE (D-Nev.) in their respective states on Friday.

"The DSCC has already begun to shift momentum in our favor in critical races — but Republicans only need to win 10 of the 15 seats up for grabs to take the majority," Obama wrote Monday. "But if you keep moving forward in the face of difficulty, we will win this election."